Today, optical sheets such as light diffusing sheets for diffusing a light from a source such as a light guiding panel and light condensing sheet (lens sheets) for condensing lights in the forward direction are used for electronic display applications such as liquid crystal display elements and organic EL displays. In many such cases, various optical sheets are stacked one over another. Japanese Patent Application Laid-Open No. 2004-184575, for instance, proposes a semi-transmissive/semi-reflective polarizing film formed by stacking a reflective polarizing sheet, a phase difference sheet and a semi-transmissive/semi-reflective layer in any desired order and further stacking absorptive polarizing sheets outside these three layers. As many as five sheets intervene between the light source device and liquid crystal cells, and this configuration is claimed to help enhance the screen luminance or reduce power consumption.
Japanese Patent Application Laid-Open No. 5-173134 discloses a light control sheet integrating both the light condensing function and the light diffusing function by stacking a light diffusing sheet and a light condensing sheet.
Japanese Patent Application Laid-Open No. 6-109925 describes increasing the luminance within the visual angle of the LCD device by the use of an optical film having a first surface which has a corrugated structure having a plurality of isosceles triangular prisms, arranged side by side, and a second surface having an optical convex-concave face which diffuses and transmits light. Japanese Patent Application Laid-Open No. 7-239472 discloses a film lens for area light source representing an attempt to enhance the luminance in the normal direction and uniformize the luminance distribution by appropriately designing the shape of the film lens on the output side of the area light source. Thus, luminance improvement and uniformization are also sought by a shape-oriented approach.